1. Field of the Invention
The present invention relates to an image-forming device, and particularly to a mechanism for controlling a fixing heater in the image-forming device.
2. Related Art
An image-forming device generally includes a fixing unit for fixing a toner image to a recording medium after the image has been transferred thereon. The fixing unit is configured of a fixing roller including a fixing heater inside, and a pressure roller that contacts the fixing roller with pressure. The recording medium is pinched between and conveyed by the fixing roller and pressure roller with the surface of the recording medium with the toner image facing the fixing roller. The heat from the fixing roller melts and fixes the toner image to the recording medium.
However, the fixing heater sometimes happens to reach an excess high temperature due to any unexpected reasons. Japanese patent application publication No. HEI 10-307514 discloses an image-forming device which cuts off electric power fed to the fixing heater when the temperature of the heater exceeds a predetermined level. Therefore, the fixing unit is protected from overheat.
One attempt to protect the fixing unit from overheat is to control the fixing heater according to a software program when the temperature of the fixing heater reaches an unusual level. However, such a software process is not as reliable since the software may happen to be out of control. On the other hand, one may control the temperature of the fixing unit by a hardware configuration. However, a problem may arise that the hardware configuration may malfunction.